Alkylene oxides (vicinal epoxy alkanes), and particularly propylene oxide, are widely used chemicals. The alkylene oxides have been polymerized with a wide variety of monomers to yield polymers which are useful in coating compositions and in the manufacture of molded articles such as urethane foams. They are also reacted with alcohols to yield monoalkyl ethers which have utility as solvents in many commercial processes and which are useful as components for synthetic turbojet lubricants.
Many methods to produce propylene oxide are known throughout the art. One method, referred to as the chlorohydrin process, involves the reaction of chlorine and water to form hypochlorous acid which is then reacted with propylene forming propylene chlorohydrin. The propylene chlorohydrin is then dechlorinated to yield propylene oxide.
U.S. Pat. Nos. 4,785,123 and 4,943,643 to Pennington, disclose vapor phase oxidation of olefins by bubbling the gases through a molten nitrate salt catalyst. The salts are a mixture of potassium and sodium salts containing 20-80 wt. % sodium nitrate. The presence of these salts serves as an isothermal medium which absorbs large quantities of heat generated during the exothermic oxidation reaction.
Non-catalytic oxidation reactions have also been disclosed. Co-pending U.S. patent application 07/620,675 by Fullington, filed Dec. 3, 1990, discloses the direct oxidation of propylene with oxygen. The operating temperature is from 100.degree. C. to 300.degree. C. at a pressure above 300 psia (pounds per square inch absolute).
U.S. Pat. No. 2,530,509 by Cook discloses reacting propane and propylene with oxygen in a plug flow reactor having a large surface area relative to the volume occupied by the reacting gases. The large surface area is required to remove heat generated during the oxidation reaction. While the direction of gas flow may be reversed, there is no suggestion of circulating the gases to obtain an isothermal reaction zone.
U.S. Pat. No. 3,132,156 to Lemon et al. discloses a reaction vessel for the oxidation of propylene which provides substantial homogeneity of reactants and essentially isothermal conditions throughout the reaction zone. The reaction temperature is maintained within the range of 425.degree. C. to 575.degree. C.
U.S. Pat. No. 4,992,567 to Meyer et al discloses the vapor phase oxidation of an alkane or olefin in molten salt catalyst in the presence of an aldehyde normally formed in the oxidation process, which aldehyde is separated from the reaction product and recirculated back into the oxidation phase reaction for co-oxidation to form carbon-oxygen gases while increasing the olefin selectivity to alkylene oxide. Reaction temperatures are in the range of about 135.degree.-600.degree. C. and the reaction pressures are in the range of about 1-50 atmospheres.
Non-catalytic direct oxidation has advantages over catalyzed oxidation or processes requiring intermediate reaction steps. There are fewer process steps to monitor and fewer chemical components to maintain, both of which reduce cost. However, until now, non-catalytic direct oxidation has been limited by low yield and poor propylene oxide selectivity. Propylene oxide selectivity is the molar percentage of propylene oxide produced for every mole of propylene which reacts within the reactor vessel.